There are a number of recent developments relating to polymers made using catalysts having a single or uniform active site such as a metallocene catalyst or constrained geometry catalyst. One class of polymers made using these types of catalysts systems is polyethylene, typically low density polyethylene. In some cases the polymers may contain long chain branching. The present invention relates to high density copolymers of ethylene.
Representative of the above art are Exxon's U.S. Pat. Nos. 5,382,630 and 5,832,631 both issued Jan. 17, 1995 to Stehling et al. The patents disclose blends of linear low density polyethylene. The blend per se has a polydispersity (Mw/Mn) greater than 3 while each component in the blend has a polydispersity of less than or equal to 3 and a different average molecular weight. The 631 patent is restricted to blends having a density from 0.88 to 0.900 g/cm3, which is well below the density of the blends of the present invention. The 630 patent claims blends having a density greater than 0.900 g/cm3, preferably from 0.900 to 0.940 g/cm3, which is essentially free of components having a higher average molecular weight and lower average comonomer content than that of any other component in the resin. This later feature is essentially directed to “reverse comonomer incorporation”. Typically with conventional catalysts at a higher molecular weight there is a reduced tendency for the incorporation of the comonomer. (Or put the other way, the comonomer incorporation tends to be higher in the low molecular weight component of the polymers.) Additionally, the references disclose polymerizations at relatively low temperatures, such as gas phase reactions at temperatures from 50° C. to 120° C. and slurry polymerizations at temperatures up to 110° C. The references do not disclose polymerizations at temperatures greater than 120° C.
Applicants have been unable to locate any art disclosing the use of multimodal resins in the manufacture of pipe.
The EXXON patents disclose producing the blends “in situ” by reaction with a metallocene catalyst. Interestingly the patents teach the polymer may be made using a gas phase, high pressure, slurry or solution polymerization. However, the reference fails to teach a dual reactor process in which the reactors are operated at different temperatures, nor does the reference suggest polymerization temperatures greater than 120° C.